All of the existing radiation imaging inspection systems for cases or luggage employ an X ray machine as a radiation source, high voltage of which is from 100 to 200 kV. Currently, the widely used X ray inspection systems for cases obtain a projection picture of a case through line scanning. In the projection picture, the images of the articles inside the case overlap on each other, while the grayscale of pixels of the images depends on the total mass thickness along the projection path. It is difficult to distinguish the material property of the articles from the projection picture of the case provided by such a DR type radiation imaging inspection system, thus it is insufficient to satisfy the requirement for searching the explosive, drug and flammable dangerous articles. In order to overcome this drawback, some major airports in USA begin to install a new radiation imaging case inspection device (disclosed by U.S. Pat. No. 5,182,764 and U.S. Pat. No. 5,367,552). Actually, the inspection device is in the form of tandem combination of the above-mentioned DR type X ray inspection system with the CT type X ray inspection system, which distinguishes the material property of the articles through the density based on the tomographic image achieved by the X-ray CT system. In order to increase the pass-through rate, the projection picture of the case is first obtained from the X ray DR inspection system, then it is determined which portions of the case require a further CT inspection based on the projection picture, finally the X-ray CT inspection system is adopted to obtain the tomographic images for these portions so as to search the illicit or dangerous articles such as explosive, drug and flammable articles; at the same time, if searched, mark them and generate an alarm.
Such a novel case inspection system still uses X ray machine as a radiation source, in which, the X ray machine of the CT inspection system has to rotate rapidly around the case. The major defects of this kind of X ray machine inspection system are:
1. The mean energy of X ray is low.
The high voltage of the above-mentioned X ray machine inspection system is from 100 to 200 kV, so the mean energy of resulted X-ray is limited to 30-70 keV. Such a low X ray energy leads to a poor penetration ability of the inspection system, thus no satisfactory inspection effect for relatively heavy cases can be achieved.
2. The configuration of the X ray machine is too complex and ponderous.
For the part of the CT inspection system, the main elements of the X ray machine, including a X-tube device and a high-voltage power supply, have to rotate around the object at a high speed (e.g. 720°/s) together with an array detector so as to increase the pass-through rate. However, the ponderous and complex X-tube device and high-voltage supply make it difficult to realize high rotary speed.
3. The lifetime is short
Since the operational lifetime of X-tube of CT inspection device is determined by the total complete tomographic scanning times, the higher the throughput rate (the number of inspected cases per unit time) is, the shorter the continuous working period of the X-tube is. For example, the X-tube of a CT inspection device can generally accomplish 100 thousand times of tomographic scan. Assumed that the pass-through rate is 360 cases per hour, and 3 times of tomographic scan process for each case, then the operational lifetime of X-tube will be only about 92 hours. In terms of 8 working hours per day, then the X-tube needs to be updated every 12 days, which greatly increases the operation cost and maintenance workload for such a X ray inspection system.
4. The irradiation field of the X-ray radiation is small
Limited by its generating mechanism, the space distribution of the X-ray is uneven, which has a preshoot character. Generally, the expanding angle of irradiation field of 100-200 kV X ray machine is about 42°. Therefore, in order to accommodate the X-ray irradiation field with the inspected object, the size of CT device must be large enough, which leads to the increase in both occupied land and weight of the equipment.
5. The cost is expensive
The price of the compact type high-voltage supply and X-ray tube device capable of moving rapidly together with the rotary frame is very expensive. If taking other elements into consideration, the total price of the case inspection device including the CT inspection system is nearly as high as 1 million dollars in US. This will inevitably restrict the wide application of this kind of inspecting device.